1. Field of the Invention
This invention relates to means for controlling clearance between rotating turbine parts and a surrounding shroud in a gas turbine engine.
2. Summary of the Prior Art
In an effort to maintain a high degree of efficiency, manufacturers of turbine engines have strived to maintain the closest possible clearance between rotating turbine parts and surrounding turbine shroud structure, because any gas which passes therebetween represents a loss of energy to the system. If a gas turbine were to operate only under steady-state conditions, it would be a relatively simple matter to establish the desired close clearance relationship between the rotating turbine parts and the turbine shroud. However, in reality, all turbine engines must initially be brought from a cold, standstill condition up to steady-state speed at relatively hot temperatures, and eventually return to the standstill condition. Operating conditions are even more complicated in turbine engines used to propel jet aircraft because the engines are frequently thrown into maximum acceleration or deceleration under both hot and cold engine temperature operation.
The problems in maintaining clearance between turbine blades and turbine shrouds under these conditions are caused by first, the mechanical expansion and shrinkage of the rotating turbine parts as brought about by changes in speed, and secondly, by relative thermal growth between rotating turbine blade tips and surrounding shrouds caused by differences in thermal inertia. One commonly used method of decreasing turbine tip clearance has been to properly select various materials with thermal properties that assist in matching radial growth responses at different engine operating conditions. Another method has been to actively direct and modulate variable temperature air on the outside of the turbine section of the engine. In this latter method, the air is directed on the turbine section during appropriate stages of engine operation to change the radial growth or shrinkage rate of the turbine shroud support in an effort to match the growth or shrinkage of the rotating turbine parts. These "active" clearance control systems generally require complex systems of pipes, valves, and controls to properly direct cooling air to the turbine section. The "active" system also requires significant amounts of compressor or fan air, much of which is underutilized because it is released outside the turbine section where it cannot be contained, thus causing a drain on engine performance.
It is, therefore, an object of the present invention to provide a gas turbine engine which allows decreases in clearance in a turbine section of the engine with a lesser drain on engine performance.
Another object of the invention is to control clearance between rotating turbine parts and surrounding shrouds during critical transient and steady-state phases of engine operation.
Another object of one embodiment of the present invention is to provide a turbine engine with a system that efficiently utilizes compressor air to decrease clearance between rotating turbine parts and a surrounding shroud during critical phases of engine operation.
These and other objects will become more readily apparent from reference to the following description taken in conjunction with the appended drawings.